class Solution {
public:
    int minPathSum(vector<vector<int>>& grid) {
        size_t m = grid.size(), n = grid[0].size();
        if(m == 0 || n == 0) return 0;
        vector<vector<int>> dp(m, vector<int>(n, 0));
        // 初始化
        dp[0][0] = grid[0][0];
        for(int i = 1; i < m; ++i)
            dp[i][0] = dp[i-1][0] + grid[i][0];
        for(int j = 1; j < n; ++j)
            dp[0][j] = dp[0][j-1] + grid[0][j];
        
        for(int i = 1; i < m; ++i){
            for(int j = 1; j < n; ++j)
                // 左或上两者较小的加当前的值
                dp[i][j] = min(dp[i-1][j], dp[i][j-1]) + grid[i][j];
        }
        return dp[m-1][n-1];
    }
};